1. Field of the Invention
The invention relates to a method of employing laser pulses to make bumps on a disk, and more particularly to employing multiple laser pulses at each location to make bumps on a disk.
2. Description of the Related Art
It is important that production run magnetic recording disks be tested for asperities prior to sale. When these disks are rotated in a disk drive, a magnetic head on a slider contacts the surface of a magnetic disk or rides on a cushion of air ("flies"), referred to as an air bearing, slightly above the surface of the disk (in the order of 0.075 .mu.m). Asperities which are high enough to impact the magnetic head slider during disk operation can cause failure of the drive. Accordingly, production run disks with asperities above a predetermined height (tolerance) must be ferreted out and discarded.
Production run magnetic disks are tested as part of the fabrication process for asperities by a slider which carries a piezoelectric transducer (PZT), the slider being referred to hereinafter as a PZT slider. The slider portion of the PZT slider emulates a magnetic head slider in a user's disk drive. When the production run disk is tested for asperities, the PZT slider rides on an air bearing at substantially the same distance from the disk as the magnetic head slider in the user's disk drive. The PZT slider both bends and moves as a rigid body due to the force of asperity impact. Both effects produce an electrical output, indicating asperity contact. When an asperity on the production run disk exceeds a predetermined tolerance, the PZT slider detects and indicates the asperity on a readout. The defective production run disk can then be discarded.
It is important that the PZT slider be properly calibrated to test for asperities on the production run disk. Accordingly, the PZT slider must be calibrated with bumps on a calibration disk which emulate undesirable asperities of a production run disk. The calibration of PZT sliders by prior art calibration disks with bumps has not been satisfactory since the bumps did not accurately and consistently emulate asperities on a production run disk. Presently, asperities are tested for diameters D.sub.r in the range of 15 to 25 .mu.m and a height in the range of 75 to 120 nm. It is expected that future testing will involve asperities with a diameter D.sub.r as small as 5 .mu.m and a height as low as 5 nm. When a calibration disk is fabricated it is important that asperities have a diameter tolerance of 2 .mu.m and a height tolerance of 2 nm. These tolerances have not been obtainable by the prior art.
One method of making bumps on a calibration disk is to indent the disk with a sharp instrument such as a needle. This produces a crater shaped bump approximately 200 .mu.m in diameter with a peripheral ridge that has a height above a nominal surface of the calibration disk of approximately 50 to 130 nm. The diameter of these crater shaped bumps is large and the peripheral ridge of the crater has a gradual slope from the nominal surface of the calibration disk. Another method of making calibration bumps employs jet plating of nickel onto a nickel phosphorous layer of a calibration disk surface. This method produces rounded bumps which have a diameter in the order of 50 to 1,000 um. These diameters are too large to emulate the asperities on a production run disk. Still another method of making calibration bumps on a calibration disk is to sputter tungsten or some other hard material onto the disk through a mask. These bumps which are larger in diameter than the aforementioned bumps, are made large to achieve sufficient durability and adhesion to the disk. With the small sliders used in current disk drives, the slider partly complies with the smooth, slowly rising prior art bumps. This results in an undetermined amount of interference of the slider with the bump and makes calibration of the PZT slider difficult. The search has been for a method of making smaller sized bumps on a calibration disk within the tolerances set forth hereinabove so as to emulate predetermined asperities of a production run disk.
In U.S. Pat. No. 5,062,021 Ranjan discloses a method of making crater shaped bumps on a magnetic disk using a laser pulse. These bumps roughen the relatively smooth surface of a magnetic disk to overcome stiction problems between a magnetic head slider and the disk during take-off and landing of the slider. The diameter of these crater shaped bumps is 0.8 mils and the height of the peripheral ridge of the crater above the nominal surface of the disk is in the range of 0.5 to 0.8 microinches. This diameter equates to 20 .mu.m and the ridge height equates to a range from 12.5 to 20 nm. This height range is too low for most current products, and the heights are not repeatable. Also, Ranjan does not teach glide calibration.
In U.S. Pat. No. 5,236,763 Luthi discloses a method of making elliptical crater shaped disks by impinging multiple pulses of a laser beam on a printing roller as the printing roller rotates relative to the laser beam. The elliptical shape is caused by the relative rotation, the first laser beam impinging on the printing roller at one location and the next laser beam impinging on the printing roller at a displaced second location. This can destroy the configuration of the peripheral ridge of the elliptical crater at the first location and will produce another peripheral ridge at the second location which is distorted somewhat by the remnants of the first peripheral ridge. Luthi does not disclose the height of the remaining peripheral ridge; however, the major and minor diameters of his elliptical crater shaped bump are 82 .mu.m and 60 .mu.m, respectively. The diameters of Luthi's bumps exceed the diameters of crater shaped bumps which emulate asperities on a production run disk.
In the aforementioned co-pending patent application, Ser. No. 08/150,525, a plurality of bumps are fabricated on a production run disk to overcome the problem of stiction between a disk and a magnetic head slider when the slider takes off and lands with respect to the disk. Methods are disclosed for making two types of bumps, one bump being a crater shaped bump and the other bump being a sombrero-shaped bump. The diameter of both bumps were in the order of 25 .mu.m. The highest elevation of the sombrero hat bump was a central peak that rises 25 nm above the nominal surface of the disk. The height of the peripheral ridge of the crater shaped bump was somewhat less than the height of the central peak of the sombrero hat bump. A single pulse of energy was employed to make these bumps. Exemplary laser beam fluences were 1.0 J/cm.sup.2, 1.5 J/cm.sup.2 and 2.0 J/cm.sup.2. The spot diameter was approximately 10 .mu.m and the duration of the pulse was from 0.3 to 90 ns. The crater shaped bump of this application does not emulate the sought for asperity of a production run disk since its peripheral ridge is too low. Further, the sought for bump height is not sufficiently repeatable with a single laser pulse.